Dave thanks for the link to the numbered illustration. Rudi thanks for the info about the chamber. I don't think there would have been much, if any, sound propagation with out it. I have a question about the bridge I'm planning to make from ebony. In an earlier post I might have mentioned an idea where the bottom surface of the bridge would have an L profile which would cause it to apply the force of the string tension to both the back edge of part #7 and 13 and downward pressure on part 7. I thought this configuration would make the zither bridge apply force in two directions, making it function, more like the violin saddle and bridge and guitar saddle. All thoughts would be appreciated.

Zithers are not violins and operate very differently. They are not guitars either. The soundbaord on the zither is actually the back. Zithers are low tension instruments and the plates are much thicker than violins or guitars. They have been mounting bridges on zithers the same way since the instrument came into being and it has worked. I understand the reasoning behind your L-shaped approach to the bridge but it's not necessary.
You might want to rethink using ebony for the bridge as well. Ebony is very prone to crack and chip and is not partiuclarly strong. It is hard and very close-grained which is why it is used extensively for fingerboards on many instruments but it is very brittle. Zithers rarely have ebony fingerboards and if they do, they are usually cracked. I would recommend cocobola as a better choice. It is very close-grained and very dense but also has a lot of oil in it that works very well for fingerboards. Do be careful of the dust as some people are very allergic to it. If any of this is not clear, just let me know and I'll go into more detail.

Just out of curiosity, aren't most violin/viola/cello etc. bridges made from maple?

If I recall from my cello playing days, the nut at the scroll end was often ebony or bone, and the saddle at the low end was made from ebony - but not the bridge. The bridge carries the vibrations of the strings down to the top of the instrument, then to the bass board, and on through the sound post to the instrument back .

All very true. The violin operates very differently from guitars or zithers. Violins have a soundpost which acts as a fulcrum around which the top and back move. The string is given constant energy by the bow. Guitars have a bridge which drives the top but the energy dissapates fairly quickly. The back is where the volume comes from. The top drives the air in the chamber and the flexibility of the back drives that air out the sound hole. In a zither, with bridges at both ends of the string, the entire box is flexed by the action of the strings. It is the movement of the back which is most important. The bracing on the top keeps the case from collapsing while being flexibile enough to move the air inside.
The best zithers I've played have no bracing on the back but the plates are relatively thick for a plucked instrument. Both the top and back are spruce that has been veneered. This entire approach is counter intuitive to a guitar maker. Zithers are very special animals and they don't have a lot in common with other instruments really. Yes, it has frets, but even the frets are different and require a different approach in their care. Our beloved concert zither is indeed unique among stringed instruments in general and plucked instruments in particular. Just my 2p.

I don't suppose I have a whole lot of advice to offer on this thread - quite frankly, the thought of taking a zither apart at the seams and messing with it scares me - but a few thoughts come to mind... The L profile sounds interesting. I've never seen a zither with one before, but if there is one thing about the concert zither it is that historically it has come in many shapes and forms and lends itself even today to new innovations (Take the Volkmann psalter zither for instance). So, it might be worth a try just to see what happens, and my hat's off to you for being adventurous. One point I would make is to be sure that the angle of the bridge where the right hand spends alot of time resting doesn't interfere with proper hand positioning, as well as the angle of the strings - they should manage to exit across the sound board in a straight line. If it doesn't, it may require a different playing technique altogether. Otherwise, who knows? You might be on to something. Best of luck!

Just a thought on bridge material. The violin family uses maple for the bridge and the strings are in direct contact with the maple. On the zither the bridge, at least mine and most I've seen, have a channel where a metal rod sits and the strings , under tension, push against the metal rod which transfers the string tension to the wooden channel in the bridge. That strikes me as quite different from the violin family. This leads me to wonder why an ebony bridge wouldn't be able to take the spread out force of the metal rod?

Just my two cents, but the violin family has strings which are bowed, and very rarely plucked. Thus the plucked string gives a continually diminishing energy to the bridge, while the bowed string gives a relatively constant energy.

Also, on many violin bridges I've seen, the E string (thinnest gauge) touches the bridge at an ebony wood insert in the maple. Some even have a felt pad under the E string. Both I assume to prevent the string from cutting down into the wood.

Of course, I'd assume many instrument makers taught their apprentices to "do it my way", and not to experiment. When the apprentice becomes a master, he/she is free to decide.

I no longer repair violins, but my gut (no pun intended!) tells me that overall, there is more "tension" on the zither strings, and thus more downward pressure, than on the violin strings.

Why not make your own zither bridge out of maple? See how it works for you, and let us know the results.